Яндекс

Saturday, April 1, 2017

Major modification of frame buffer and video interface emulation.

I already wrote about N64 Video Interface emulation in GLideN64. It was my first attempt to make things right. Three years passed. Many elements of frame buffer emulation mechanism have been modified since that time. However, one major problem remained. This problem is as old as N64 emulation itself. This is "frame buffer height" problem.

To render anything you first need to allocate rectangular buffer, which will hold your graphics. You need to know width and height to allocate the buffer. The problem is that RDP command SetColorImage set only width of color buffer. Height is not set. RDP does not need to know buffer height. SetColorImage provides buffer origin, number of pixels per line and size of each pixel in bytes. This is enough to calculate position of vertex with given X and Y coordinates within the buffer. Scissor command prevents out of buffer writes. Software graphics plugin works exactly as RDP and also does not need to know buffer height. Hardware plugin is in trouble. Suppose, we selected 960x720 resolution with 4:3 aspect ratio and created 960x720 render buffer in video memory. N64 game allocates buffer with width 320. Which scale should we apply to original N64 coordinates to get correct picture in our render buffer? Since 960 = 3 x 320, it seems that correct scale is 3x. That is we scale original N64 X and Y coordinates by 3 and get picture in our buffer. Will this picture be correct? Only if original buffer also has 4:3 aspect, that is has size 320x240. In reality, it also can be 320x220, 320x256 or even 320x480. In all these case 3x scale for Y will give us wrong result. To get correct Y scale we need to know height of original buffer, but it is not available.

Height of N64 render buffer can be estimated from parameters of Video Interface, which defines how color buffer will be mapped to TV screen. All hardware plugins, which I know from inside use this possibility. Thus, frame buffer allocation becomes dependent on VI registers. This dependency does not exist in N64 itself. The height estimation does not guarantee to be always correct, and in fact it is often incorrect. The estimation code is complex and full of heuristics, to reduce numbers of errors. Nevertheless, this tie still induce many issues, in particular with PAL games and with games, which use interlaced TV modes.

Besides main color buffers, whose content is displayed on TV, N64 games often use auxiliary color buffers. These buffers are used for variety of purposes: dynamic shadows, reflections, TV monitors and so on. Auxiliary color buffer can be of any size. Thus, estimation of auxiliary buffer height is complex and fully heuristic algorithm, which also not always works right. Wrong height lead to visual glitches.

At the end of 2016 I finally invented the way to get rid of necessity to know exact height of N64 color buffers. The idea is actually very simple. Why RDP does not care about buffer height? It knows that the height is large enough and just fills the buffer with primitives. Video Interface takes necessary part of the buffer and maps it on TV screen. Auxiliary buffers are used as textures: game's program code knows buffer's bounds and maps texture coordinates to its content.
My frame buffer mechanism creates separate frame buffer object in video memory for each buffer allocated by RDP. I used estimated height to create the buffer render target. It caused aforementioned issues when estimation heuristics failed and produced wrong result. So, the idea is to not use estimated buffer height and always use large enough height instead. 'Large enough' should be taken literally. It is some value, which is surely greater or equal to any possible height of N64 buffer. There are some natural limitations: maximal buffer size for NTSC is 640x480 and 640x576 for PAL.Since I know width of rendering resolution selected by user and I know width of N64 rendering buffer - I know how to scale original coordinates of N64 vertices. This scale can be applied for X and Y coordinate, no matter has the N64 buffer the same aspect as user selected screen resolution or not. Video Interface emulation will map my frame buffer object to screen the same way as N64 Video Interface maps N64 buffer in RDRAM to TV screen.

Pros:

No more buffer height estimation heuristics.

No more glitches caused by wrong height estimation

Emulation of effects, not working before

Cons:

More video memory needed. Memory overhead is not large for main buffers, because actual buffer height is usually close to natural limit used as Large Enough Height. Memory allocated for auxiliary can be 10 times more than actually used.

While the idea is simple, its implementation was not. It was obvious, that lots of things need to be changed. The first step was code refactoring, mentioned in the previous article. After that step I got more clear and easy to modify code. It was not enough though. Some preliminary steps had to be done first.

There is one OpenGL specific problem with emulation of N64 graphics. N64 uses coordinate system with origin in upper left corner. Glide3X API allowed to set origin to either upper left or to lower left. So, when I worked on Glide64, I set origin to upper left and had no inconveniences. OpenGL has origin nailed to lower left corner. If you will use N64 coordinates, you will get image upside down. Thus, Y coordinate must be inverted. (0,0) coordinate translated to (0, maxY), where maxY is buffer's height.

It is simple trick, but you need to apply it everywhere: modify vertex Y, viewport Y, scissor Y. Read from frame buffer to RDRAM have to be done in reverse order. Things could get even more complicated with new frame buffer technique. Thus, I decided to remove Y inversion. Of course, image will be upside down in that case.

However, the image is in frame buffer texture, which I can map to screen as I need. So, it is not a problem. The problem arises when you do not use frame buffer object and do rendering right to back buffer. GLideN64 renders right to screen when frame buffer emulation disabled. I did not want to keep Y inversion code to support "no frame buffer emulation" mode. My goal was to simplify things, not to make them more complex and intricate. Thus, I decided to slightly modify "no frame buffer emulation" mode: use one frame buffer object for rendering instead of direct render to back buffer. It also mentioned in previous article: "Anti aliasing without frame buffer emulation". After that modification I could safely remove Y inversion code.

After preliminary work completed, real challenge started. Implementation of my idea was a very hard task. Frame buffer emulation was twisted tight with VI emulation, and I spent many time untangling multiple knots and fixing weirdest glitches. At the end I was totally rewarded. Issues with cut image in PAL games gone. Issues with screen shakes in interlaced mode gone. Many crashes with buffer copy to RDRAM gone. VI effects started to work more smooth. Screen shrink VI effect in Mia Hamm Soccer finally start to work properly.